This project is part of a Program Project that is a collaborative research effort to design highly selective opioid receptor peptide or peptidomimetic ligands for the nonaddictive treatment of pain and drug dependency. The objective of this project is to elucidate which molecular characteristics of opioid receptor ligands may be modified to increase their passive transport from the blood to receptor sites in the brain. This necessarily involves permeation through biological membranes that compose the blood-brain barrier. In order to understand the passive permeation of these ligands through membranes, a biophysical approach will be used to analyze the permeation in terms of the water-to-membrane partition, transmembrane diffusion, and membrane-water interfacial interactions. These studies include both the determination of the rate of peptide permeation as a function of the peptide structure and the membrane composition; and the characterization of the thermodynamics of ligand transfer from water to the membrane. The later includes the determination of partition coefficients via an equilibrium dialysis procedure developed for these peptides, and isothermal titration calorimetry to directly measure the enthalpy of peptide-membrane binding. Taken together these data provide the necessary information to ascertain which elements of the peptide structure may be usefully modified to enhance the peptide permeation across membranes. Over the course of the previous studies the membrane permeability of opioid peptides has been increased by two orders of magnitude. Among the most permeable of these new opioid ligands are analogues of the potent analgesic, biphalin. However, the greatest impediment to achieving high membrane permeability with biological membranes is the electrostatic interactions of cationic peptides, such as biphalin, with anionic membranes. A major goal of the proposed research is to enhance opioid ligand permeability across anionic membranes through the design of peptides that retain biological activity, exhibit high partition coefficients and diffusion coefficients, yet minimize the effect of electrostatic interactions at membrane surfaces.